Offshore transfer system with a docking position on a transfer vessel that comprises a motion compensated mooring element

ABSTRACT

An offshore transfer system for transferring persons and/or cargo between a transfer vessel and an offshore object, includes the transfer vessel having a docking position with a mooring element. The offshore object includes a docking arm, in particular a gangway, that at a free end is provided with a coupling device. The docking position on the transfer vessel has its mooring element at least partly compensated for transfer vessel motions, at least during a docking operation, and includes one or more force actuators for moving the mooring element relative to the transfer vessel, one or more sensors for detecting motions of the transfer vessel, and a control unit for driving the one or more force actuators such that the motions of the transfer vessel get at least partly compensated for the mooring element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Netherlands Application No.2024859, filed Feb. 7, 2020, the contents of which is incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to an offshore transfer system fortransferring persons and/or cargo between a transfer vessel and anoffshore construction, floating platform or other vessel, wherein thetransfer vessel is provided with a docking position, wherein theoffshore construction, floating platform or other vessel comprises adocking arm, in particular a gangway, that at a free end is providedwith a coupling device, and wherein the docking position comprises amooring element that is configured to have the coupling device at thefree end of the docking arm coupled thereto during a docking operation.

BACKGROUND TO THE INVENTION

From the state of the art all kinds of systems and constructions areknown for making temporary connections at sea between transfer vesselsand offshore constructions, floating platforms or other vessels. Thosetemporary connections particularly get formed by gangways. After such agangway has been reliably coupled during a docking operation, thepersons and/or cargo can then be quickly and safely transferred from thetransfer vessel towards the offshore construction, floating platform orother vessel, and vice versa.

The gangway that mostly is provided on the transfer vessel, is providedat its free end with a suitable coupling device. This coupling device isconfigured to couple with a complementary mooring element of a dockingposition that forms part of or that is specifically provided on theoffshore construction, floating platform or other vessel.

For example, from WO-02/20343 a system is known that is used forachieving a flexible walkway connection between a vessel and an offshoreconstruction. For this purpose, the vessel is provided with atelescopically extendable walkway which at one end is movably mounted onthe vessel around two shafts. At the free end of the walkway a couplingdevice is provided, which is made in such a way that it can be coupledto a substantially vertically pointing grip bar connected to theoffshore construction. While mooring, the vessel is manoeuvred to asuitable starting position in relation to the grip bar. Next, thewalkway is aimed towards the middle part of the grip bar by means of asuitable swivel movement and the walkway is extended until the couplingdevice encompasses the grip bar. Then, the coupling device's twohydraulically controlled coupling jaws—which can move towards each otherfrom an open position—are closed. They are closed in such a way, thatthe coupling device still has the freedom to move upwards along the gripbar during the coupling phase. After reaching the required couplingposition just below the landing, the coupling claws are moved closertowards each other in order to tightly clamp onto the grip bar and fixthe position at that location. Finally, a number of degrees of freedomof the walkway are released, i.e. the movability around the two shaftsand the telescopic extendibility. By using rotatable screws andsatellite navigation, the vessel can be kept at its location withoutstrong forces being submitted to the walkway and/or the grip bar.

A disadvantage here, is that the coupling procedure can sometimes besomewhat cumbersome, especially during stormy weather. The forces thatare generated at the moment that the arm hits the grip rod, are veryhard to control under these circumstances. This is due to the fact thatat the moment of contact, large weights have to be slowed down and thedrives concerned must be switched off immediately at the same time.Additionally, during the coupling phase, closing the coupling jaws hasto be accurately attuned to the release of the walkway's remainingdegrees of freedom, in order to avoid damages to the vessel. Anotherdisadvantage is that the vertical grip rod has to extend over a largerange, making it relatively expensive and difficult to assemble.Furthermore, the known system proved to be unsuitable for smallervessels that are not ‘dynamic positioned’, i.e. they are not equipped insuch a way that they can be kept in position by means of rotatablescrews and satellite navigation.

Various other docking positions with mooring elements are known from thestate of the art to be provided on all kinds of offshore constructions,floating platforms or vessels, and that are all specifically configuredto cooperate with specifically designed coupling devices at the ends ofgangways or other types of docking arms that are provided on all kindsof transfer vessels.

For example, Ampelmann nowadays is using a rubber D-buffer that ismounted at a tip of a gangway as coupling device, and a U- or H-profilethat is mounted at a docking position of an offshore construction asmooring element. The rubber D-buffer is destined to be pushed into theU- or H-profile during a docking operation.

Here however the same disadvantage goes that the coupling phase leavesto be improved because it can be somewhat time consuming, less reliableand/or bring along a risk that vulnerable parts of the docking positionor of the gangway may get damaged during the coupling phase,particularly during harsh weather or rough sea, or e.g. because one ormore degrees of freedom of the gangway are difficult to control to staystationary relative to the mooring element and/or are not released intime.

BRIEF DESCRIPTION OF THE INVENTION

The present invention aims to overcome those disadvantages at leastpartly or to provide a usable alternative. In particular the presentinvention aims to provide a user-friendly and safe offshore transfersystem that is able to make the docking operation itself easier, quickerand less vulnerable, and that is also able to be properly performedduring harsh weather conditions, rough sea or the like that cause thetransfer vessel and possibly also the floating platform or other vesselto ride the waves and make large pitch, roll and/or heave motions.

According to the present invention this aim is achieved by an offshoretransfer system for transferring persons and/or cargo between a transfervessel and an offshore construction, floating platform or other vesselaccording to the present invention. The transfer vessel according tothis system comprises a docking position, whereas the offshoreconstruction, floating platform or other vessel according to this systemcomprises a docking arm, in particular a gangway, that at a free end isprovided with a coupling device. The docking position on the transfervessel comprises a mooring element that is configured to have thecoupling device at the free distal end of the docking arm coupledthereto during a docking operation. According to the inventive thoughtthe docking position on the transfer vessel has its mooring element atleast partly compensated for motions of the transfer vessel, at leastduring the docking operation. For this, the docking position comprisesone or more force actuators for moving the mooring element relative tothe transfer vessel, one or more sensors for detecting one or more ofpitch, roll and/or heave motions of the transfer vessel, and a controlunit for driving the one or more force actuators such that the one ormore detected motions of the vessel get at least partly compensated forthe mooring element of the docking position.

Advantageously, the invention is able to create a stable mooring elementof the docking position on the transfer vessel, even when this transfervessel is thrown in all kinds of directions by wind and waves. Owing tothe provision of the feedback-loop between the sensors, the control unitand the force actuators, it is possible to keep the mooring elementsubstantially still/motionless in one or more orientations, or whendesired even in its entirety.

During compensation, the one or more force actuators of the dockingposition on the transfer vessel are accurately driven by the controlunit in dependency of the one or more of pitch, roll and/or heavemotions of the transfer vessel that are detected by the one or moresensors, in such a way that those one or more detected motions get atleast partly compensated for the mooring element. In this manner one ormore of x-, y- and z-orientations of the mooring element can bemaintained independent of the one or more motions of the transfervessel. Thus the mooring element can be kept quite stable for thoserespective one or more x-, y- and z-orientations, while the transfervessel has all freedom to make all kinds of small or large pitch, rolland/or heave motions that are caused by waves, wind, etc.

This may truly facilitate the coupling of the coupling device at the endof the docking arm, like the gangway, thereto during a docking operationof the transfer vessel with the offshore construction, floating platformor other vessel. Either the docking arm can be quickly, easily andreliably be manoeuvred with its end part towards the stably held atleast partly motion compensated mooring element, either the transfervessel with its stably held at least partly motion compensated mooringelement can be quickly, easily and reliably be manoeuvred towards theend part of the docking arm. A large risk for dangerous or possiblydamaging situations to occur is no longer present.

Another important advantage is that the invention also makes it possibleto maintain having the mooring element at least partly motioncompensated after the coupling with the docking arm has taken place.This may facilitate a stepping from the transfer vessel onto the gangwayand from the gangway onto the transfer vessel.

In an embodiment the compensation of the mooring element is controlledsuch by the control unit that the position resp. orientation of themooring element is referred to the “fixed world”, that is to say keptsubstantially stable relative to the horizon. It is however alsopossible that the compensation of the mooring element is controlled suchby the control unit that the position resp. orientation of the mooringelement is referred to the coupling device at the end of the dockingarm, including when this docking arm is of a type that can be activelymanoeuvred such that the coupling device at its free end gets to makemovements to and from the mooring element during a docking operation.The at least partly motion compensated mooring element is well able toalso take care of compensating any such movements of the free end of thedocking arm as well. For that, one or more additional sensors can beprovided for detecting movements of the free end of the docking arm aswell, and the control unit for driving the one or more force actuatorsthen can be configured such that the movements of the free end of thedocking arm also can get at least partly compensated for the mooringelement of the docking position on the transfer vessel.

In addition thereto or in the alternative, when the docking arm isprovided on a somewhat moving floating platform or other vessel, thedocking arm advantageously may be supported thereupon via a basis thatdoes not necessarily have to be motion compensated relative to thisfloating platform or other vessel at all, not even partly. The at leastpartly motion compensated mooring element is well able to also take careof compensating any such movements of the floating platform or othervessel as well. For that, one or more additional sensors can be providedfor detecting one or more of pitch, roll and/or heave motions of thefloating platform or other vessel as well, and the control unit fordriving the one or more force actuators then can be configured such thatthe one or more detected motions of the floating platform or othervessel also can get at least partly compensated for the mooring elementof the docking position on the transfer vessel.

Owing to the invention, the docking arm, during a docking operation, maybe kept rigid in its entirety relative to the offshore construction,floating platform or other vessel upon which it is supported. Thetransfer vessel with its stably held at least partly motion compensatedmooring element then can be sailed towards the offshore construction,floating platform or other vessel, where it can be carefully manoeuvredwith its stably held at least partly motion compensated mooring elementtowards the coupling device at the free end of the docking arm and haveit coupled thereto.

It is however preferred that the docking arm is of a type that, during adocking operation, can be actively manoeuvred along one or more degreesof freedom, like being swivelled around a horizontal axis and/or turnedaround a vertical axis at the location of its supported proximal endrelative to the offshore construction, floating platform or othervessel, and/or like being extended or retracted by movable telescopingparts thereof. The stability in space of the mooring element on thetransfer vessel then makes it way easier for an operator to manoeuvrethe docking arm with its coupling device towards the mooring element andcouple it therewith. It even makes it possible to have such a couplingoperation performed in a fully automated manner without an operatorbeing needed at all. Another important advantage of an at least partlymotion compensated mooring element that is provided on a transfer vesselin combination with a manoeuvrable docking arm that is provided on anoffshore construction, floating platform or other vessel, is that it isno longer critical during a docking operation, when the docking arm'sdegrees of freedom get released. This release no longer has to beaccurately attuned with the exact moment that the coupling device hasgotten coupled with the mooring element. As long as the mooring elementgets at least partly compensated for the one or more motions of thetransfer vessel underneath it, no large pushing or pulling “break-free”forces shall all of a sudden be able to start occurring. This adds tothe safety of the offshore transfer system. The same goes for adecoupling phase, then also it is no longer critical at what exactmoment in time the docking arm needs to get actively manoeuvred again inrelation to the exact moment in time its coupling device gets activelyreleased again from the mooring element on the transfer vessel.

The transfer vessel with its stably held at least partly motioncompensated mooring element then can be sailed towards the offshoreconstruction, floating platform or other vessel where it can be kept‘dynamic positioned’. Subsequently the docking arm can be carefullymanoeuvred with the coupling device at its free end towards the stablyheld at least partly motion compensated mooring element and have itcoupled thereto.

In a preferred embodiment, the mooring element can be supported by andconnected to the transfer vessel by means of a hinge connection that isdesigned to allow the transfer vessel to rotate relative to the mooringelement in a rotation direction around an y-axis, wherein the y-axisextends substantially horizontal, in particular in a directiontransverse to a longitudinal direction of the transfer vessel, andwherein a first one of the one or more force actuators acts between thetransfer vessel and the mooring element for swivelling the mooringelement around the hinge connection relative to the transfer vessel atleast in the rotation direction around said y-axis. Thus the mostimportant one of the transfer vessel motions, that is to say the pitchrotations around its transverse y-axis can get compensated for.

In addition thereto or in the alternative, the hinge connection can bedesigned to allow the transfer vessel to also rotate relative to themooring element in a rotation direction around an x-axis, wherein thex-axis extends substantially horizontal, in particular in a directionparallel to a longitudinal direction of the transfer vessel, wherein thex- and y-axis extend orthogonal relative to each other, wherein a secondone of the one or more force actuators acts between the transfer vesseland the mooring element for swivelling the mooring element around thehinge connection relative to the transfer vessel at least in therotation direction around said x-axis. Thus the second most importantone of the transfer vessel motions, that is to say the roll rotationsaround its longitudinal x-axis can get compensated for.

In combination a specific type of 2-DOF hinge support of the mooringelement on the transfer vessel can be obtained that together with theprovision of the force actuators acting in those same 2-DOF'sadvantageously make it possible to provide a well-supported mooringelement, of which a desired orientation, can be maintained under allcircumstances whatever pitching or rolling movements the transfer vesselmakes on the waves. Each time the mooring element leaves its desiredorientation, the force actuators can be actuated in such a way that themooring element is quickly and smoothly brought back into its desiredorientation. The force actuators do not have to bear the weight of themooring element, this weight can be carried by the hinge connection. Theforce actuators merely serve the purpose of keeping the mooring elementin its desired orientation.

The hinge connection is of the type having at least two rotationaldegrees-of-freedom (DOF) whereas shifting displacements between thetransfer vessel and the mooring element at the location of the hingeconnection are counteracted, and whereas forces, like weight forces, canbe transmitted from the mooring element towards the vessel and viceversa. The hinge connection can take up forces not only in a downwarddirection but also in the sideways directions without allowing relativedisplacements of the hinge connection itself in those directions. Thehinge connection thus forms a true 2-D pivotal connection between themooring element and the transfer vessel.

Persons and cargo can now be transferred over the docking arm towardsthe offshore construction, floating platform or other vessel. Whencoupled to the transfer vessel, the docking arm is supported at bothends between the transfer vessel and the offshore construction, floatingplatform or other vessel, and forms a reliable bridge between them, forexample in the form of a gangway. The permanent stable positioning ofthe mooring element herewith helps to prevent or minimize uncontrollableacceleration forces in horizontal directions caused by sudden pitchingor rolling transfer vessel movements. Such forces could otherwise leadto uncontrollable situations in which the docking arm might get damaged.

In a particular embodiment the hinge connection may comprise twosubstantially horizontal orthogonal pivot pins. Those two pivot pins mayform part of a so-called Universal or Cardan joint or an assembly of twoGimbals also referred to as a two-axis Gimbal. The Universal joint,Cardan joint or Gimbal assembly allows the mooring element and transfervessel to swivel around both the x- and y-axes relative to each other.The pivot pins are located close together and preferably lie in a samecommon substantially horizontal plane. They form a double-pivotedsupport that allows the rotation of the mooring element and vesselrelative to each other about two respective axes. With the Universaljoint, the Cardan joint and the Gimbal assembly, the mooring element canbe mounted on one of the pivot pins, whereas the transfer vessel can bemounted to the other remaining pivot pin. Together the pivot pins of thehinge connection are well able to keep the mooring element substantiallyimmobile with respect to its orientation relative to the horizonregardless of the pitching and rolling motions of the transfer vessel.

In a preferred further or alternative embodiment, the mooring elementmay be supported by and connected to the transfer vessel by means of az-guidance designed to allow the transfer vessel to move relative to themooring element in a translation direction along a z-axis, wherein thez-axis extends substantially vertical, and wherein a third one of theone or more force actuators acts between the transfer vessel and themooring element for translating the mooring element along the guidancerelative to the transfer vessel at least in the translation directionalong said z-axis.

In addition thereto or in the alternative, the mooring element may besupported by and connected to the transfer vessel by means of anx-guidance designed to allow the transfer vessel to move relative to themooring element in a translation direction along the x-axis, wherein afourth one of the one or more force actuators acts between the transfervessel and the mooring element for translating the mooring element alongthe guidance relative to the transfer vessel at least in the translationdirection along said x-axis.

In addition thereto or in the alternative, the guidance can be designedto allow the transfer vessel to also move relative to the mooringelement in a translation direction along the y-axis, wherein a fifth oneof the one or more force actuators acts between the transfer vessel andthe mooring element for translating the mooring element along theguidance relative to the transfer vessel at least in the translationdirection along said y-axis.

Thus any substantial vertical up and down movements and/or anysubstantial horizontal back and forth movements and/or any substantialleft and right movements of the transfer vessel no longer have to beabsorbed by the docking arm changing its angle of inclination around ahorizontal hinge connection and/or around a turntable connection withthe offshore construction, floating platform or other vessel. Insteadthe mooring element itself then can get actively compensated for thoseup and down, back and forth, and left and right movements as well by thecontrol unit and the respective force actuators.

In a preferred further or alternative embodiment, the docking positionfurther may comprise an upright column extending upwards from thetransfer vessel, wherein the mooring element is provided on an upperportion of the column. This adds to the safety of the transfer systemparticularly during storm and high waves.

In addition thereto, lever arms can then be provided which are fixedlyconnected to the column and extend in a substantially sideways directionfrom it, the force actuators being provided in between the transfervessel and the lever arms. The lever arms make it possible to use lessstrong force actuators.

The force actuators can be of various types, like spindles, stays whichcan be tensioned or loosened, etc. Preferably, hydraulic cylinders areused since they are able to respond very quickly and thus are able toprevent the mooring element, or the column on top of which the mooringelement can be provided, from swinging around too much.

Preferably the control unit operates fully automated. For this aposition sensor for sensing the orientation of the mooring elementrelative to the horizon, in particular for sensing deteriorations of thecolumn, on top of which the mooring element preferably gets mounted,from a substantially vertical orientation. Furthermore the control unitthen is provided for automatically steering the force actuators independence of sensed orientations by the position sensor in such a waythat the force actuators bring the mooring element back to its aimedorientation, in particular bring the column on top of which it ismounted, back to its substantially vertical orientation. Depending onthe rolling and pitching speed of the transfer vessel, the column inthis way can for example be kept within +/−2 degrees from the verticaluntil wind-force 8.

Further preferred embodiments of the invention are described herein.

The invention also relates to a transfer vessel for an offshore transfersystem having a mooring element that is at least partly compensated fortransfer vessel motions and to methods for coupling a transfer vesselwith the docking arm on the offshore construction, floating platform orother vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the transfer system according to the invention shall beexplained in more detail below with reference to the accompanyingdrawings in which:

FIG. 1 schematically shows a perspective view of a motion compensatedmooring element on a transfer vessel together with part of a gangway tobe coupled thereto;

FIG. 2 shows a first variant hereof; and

FIGS. 3a and 3b show a second variant hereof preceding and aftercoupling.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a transfer vessel, of which merely a small portion is shown,is given the reference TV. The transfer vessel TV has a longitudinaldirection along an x-axis and a transverse direction along a y-axis. Thetransfer vessel TV is provided with an upper deck on top of which adocking position DP is provided that comprises a supporting column SCand a mooring element ME on top thereof. The column SC extends upwardlyin a vertical direction along a z-axis.

Between the column SC and the transfer vessel TV a Cardan joint 4 isprovided. The Cardan joint 4 forms a hinge connection which has twopivot pins 4 a, 4 b extending in the horizontal directions x and y andallowing the transfer vessel TV and the column SC to rotate relative toeach other around those x- and y-axes. An angle of 90 degrees isenclosed between the two pivot pins 4 a, 4 b.

The pivot pin 4 b is supported by brackets 5 which are fixedly connectedto the transfer vessel TV. The pivot pin 4 a is supported by brackets 6which are fixedly connected to a lower end of the column SC. The columnSC projects upwardly in the vertical direction along a z-axis andencloses angles of 90 degrees with both pivot pins 4 a, 4 b.

The lower end of the column SC includes two lever arms 10, a first one10 a extending in the x-direction, and a second one 10 b extending inthe y-direction. Between outer ends of the lever arms 10 and thetransfer vessel TV, first and second hydraulic cylinders 11 a, 11 b areplaced. Those cylinders 11 may be pivotally mounted with one side to thelever arms 10 and at their other side to the transfer vessel TV. Whenoperated, the hydraulic cylinders 11 are lengthened or shortened andforce the column SC, together with the mooring element ME on topthereof, to rotate/swivel around the x- and/or y-axis relative to thetransfer vessel TV.

The column SC has a lower column part SCa and an upper column part SCb.The upper column part SCb is guided telescopingly relative to the lowercolumn part SCa such that it is movable up and down along the z-axis.Between the lower and upper column parts SCa and SCb, a third hydrauliccylinder 20 is placed. When operated, the hydraulic cylinder 20 islengthened or shortened and forces the upper column part SCb, togetherwith the mooring element ME on top thereof, to translate along thez-axis relative to the transfer vessel TV.

The mooring element ME is configured to have a complementary couplingdevice CD that is provided at a free end of a gangway GW coupled theretoduring a docking operation of the transfer vessel TV with an offshoreobject (not shown in FIG. 1) on which the gangway GW is provided. In theembodiment shown in FIG. 1 the mooring element ME comprises a couplingsurface 22 and two upwardly projecting pins 23, whereas the couplingdevice CD comprises a coupling surface 24 with two through goingopenings 25 that are dimensioned slightly larger than the pins 23.

Between the mooring element ME and the top of the column SC, a firstguidance member 30 is provided that is movably guided relative to theupper column part SCb such that it is movable back and forth along thex-axis. Between the first guidance member 30 and the upper column partSCb, a fourth hydraulic cylinder 31 is placed. When operated, thehydraulic cylinder 31 is lengthened or shortened and forces the mooringelement ME to translate along the x-axis relative to the transfer vesselTV.

Between the mooring element ME and the top of the column SC, furthermorea second guidance member 34 is provided that is movably guided relativeto the first guidance member 30 such that it is movable betweenstarboard and port side along the y-axis. Between the first guidancemember 30 and the second guidance member 34, a fifth hydraulic cylinder35 is placed. When operated, the hydraulic cylinder 35 is lengthened orshortened and forces the mooring element ME to translate along they-axis relative to the transfer vessel TV.

Between the mooring element ME and the top of the column SC, furthermorea turntable 38 is provided that is rotatable relative to the secondguidance member 34 such that it is rotatable clockwise andcounter-clockwise around the z-axis. Between the second guidance member34 and the turntable 38, a toothed gear 39 is placed. When operated, thetoothed gear 39 is rotated in either direction and forces the mooringelement ME to rotate around the z-axis relative to the transfer vesselTV.

A possible method for docking the transfer vessel TV to the offshoreobject shall now be explained with reference to FIG. 1. In FIG. 1 it isshown that the vessel TV has already been sailed to a position alongsidethe offshore object such that the mooring element ME is positioned inthe neighbourhood of the gangway GW that projects sideways of theoffshore object. The transfer vessel TV than be kept at that position,for example by being dynamically positioned.

The gangway GW is of a type that is telescopingly extendable and thatcan be turned around a vertical axis and that can swivel around ahorizontal axis. Thus if necessary the gangway GW can be manoeuvred topoint with its outer end towards the mooring element ME. If necessary,for example depending on the water level of the sea, it is also possibleto raise or lower the gangway GW in such a way that the gangway GW getsto point towards the mooring element ME. Subsequently the gangway GW canbe extended until the coupling device CD comes to lie straight above themooring element ME. By subsequently lowering the gangway GW, thecoupling device CD automatically gets to grip with its openings 25 overthe pins 23.

During the entire docking operation cq coupling process the mooringelement ME can be kept in a stable orientation and position relative tothe offshore object.

As soon as the coupling is made, the gangway GW can be given the fullfreedom to extend or retract such that the distance between the transfervessel TV and the offshore object can change somewhat. At the same timethe swivelling of the gangway GW can also be set free such that thegangway GW is free to alter its angle of inclination, which gives thetransfer vessel TV the freedom to rise up and downwards somewhattogether with the waves. Furthermore, the turning of the gangway GW canalso be set free such that the gangway GW is given the full freedom toturn such that the transfer vessel TV is given the freedom to turn leftor right.

According to the invention the column SC with the mooring element ME ontop thereof maintains to be vertically orientated both during couplingand after the coupling has been made. For this, operation of thehydraulic cylinders 11 may be necessary. Depending on the amount anddirection of rolling or pitching movements the transfer vessel TV makes,the hydraulic cylinders 11 need to be adjusted in length in order tohave the column SC maintain its vertical orientation. This is obtainedby means of a control unit which receives sensor signals of a positionsensor which is built into the mooring element ME. Whenever the controlunit receives a signal of the sensor that the column SC has left itsvertical orientation, it immediately sends out corresponding signals tothe hydraulic cylinders 11 to change their length(s) and with this exertsuitable forces to the column SC in order to have it move back towardsits aimed vertical orientation.

The hydraulic cylinders 11 can be operated over and over again each timethat it is necessary to make a correction in order to have the column SCmaintain its vertical orientation.

In a similar manner to the abovementioned automated compensation of theorientation of the mooring element ME for rolling and pitching movementsof the transfer vessel TV, also an automated compensation of theposition of the mooring element ME for x-y-z-translational motions,movements or displacements of the transfer vessel TV can take place,both during coupling and after the coupling has been made. For this,operation of the hydraulic cylinders 20, 31, 35 may be necessary.Depending on the amount and direction of the translational movements thetransfer vessel TV makes, the hydraulic cylinders 20, 31, 35 need to beadjusted in length in order to have the mooring element ME maintain notonly its orientation but also its position. This is also obtained bymeans of the control unit receiving the sensor signals of the positionsensor. Whenever the control unit receives a signal of the sensor thatthe mooring element has left its position, it immediately sends outcorresponding signals to the hydraulic cylinders 20, 31, 35 to changetheir length(s) and with this exert suitable forces to the mooringelement ME in order to have it move back towards its aimed position.

In addition thereto or in the alternative, the operation of thehydraulic cylinders 20, 31, 35 also can be used for docking the transfervessel's mooring element ME onto a gangway with less DOF's, for examplea gangway that is not telescopingly extendable and/or not turnableand/or not swivable. After the transfer vessel TV has been sailed to theproximity of such a more rigid gangway GW, the mooring element MEsubsequently can be accurately manoeuvred towards the coupling device CDby means of operation of the hydraulic cylinders 20, 31, 35. During thisaccurate manoeuvring, the mooring element ME is advantageouslycontrolled by the control unit to substantially maintains itsorientation by means of the automated pitch and roll compensations. Theaccurate manoeuvring can be done manually but also can be automated byequipping the mooring element ME and the coupling device CD withsuitable sensors that are configured to send signals to the controlunit. If it is then detected that the pins on the mooring element ME arenot properly aligned with the openings in the coupling device CD on thegangway GW, then the mooring element ME can even be rotated around thez-axis by operation of the toothed gear 39. Thus a coordination of themooring element ME with the stationary or moving tip of the gangway GWis possible.

In FIG. 2 a variant is shown in which the mooring element ME is onlycompensated for pitch and roll motions as well as for up and downmotions. For that the docking position (DP) still comprises atelescopingly extendable support column SC that is rotatable around ahinge connection with two horizontal orthogonal pivot pins 4 a, 4 b.

The mooring element ME here comprises an upwardly projecting bar 50above a dish-shaped coupling surface 51.

It can be seen here that the gangway GW is mounted with a base 52 to anoffshore object that here is formed by a Floating Production, Storageand Offloading (FPSO) platform. Between the base 52 and the gangway GW ahinge connection is provided which has a single pivot pin 54 whichextends in a horizontal direction and which allows the gangway GW toswivel around this horizontal axis. The gangway GW comprises a fixedgangway section GWa and a telescoping gangway section GWb. Thetelescoping gangway section GWb can slide in and out of the fixedgangway section GWa in a direction A. Between the two sections(hydraulic) drive means can be provided for actively lengthening orshortening the gangway GW whenever desired, in particular during acoupling action to the mooring element ME on the transfer vessel TV. Thegangway GW is hung to the base by means of cables 55 which are run overa hoisting device. Thus the angle of inclination of the gangway GW canactively be altered whenever desired, in particular during a couplingaction to the mooring element ME on the transfer vessel TV. Between thebase 52 and the offshore object a turnable connection is provided whichallows the base 52 together with the gangway GW to turn around avertical axis.

At its outer free end the gangway GW is provided with the couplingdevice CD that here is formed by a hook 52 which is complementary to thebar 50 and that is operable between an open and closed position.

In FIG. 3 a variant is shown in which the mooring element ME is onlycompensated for pitch motions. For that the docking position comprises asupport column SC that now is merely rotatable around a hinge connectionwith one horizontal pivot pins 4 a.

The mooring element ME here comprises a sideways projecting connectionsurface. The gangway GW here is of a similar type as the one shown inFIG. 2, that is to say that it is telescopingly extendable, turnable andswivable relative to the offshore object it is mounted to. At its outerfree end the gangway GW is provided with a coupling device CD that iscomplementary to the mooring element ME, for example by means ofelectromagnetic attraction forces.

Thus an economic simple version of the transfer system is provided thatstill makes use of the inventive pitch compensation for its mooringelement on the transfer vessel. The other transfer vessel motions can bedealt with by the slewing/luffing/telescoping provisions that arealready provided in the gangway GW on the other offshore object.

Besides the embodiments shown numerous variants are possible. Forexample the dimensions and shapes of the various parts can be varied,and instead of hydraulic cylinders other types of force actuators can beused. Also all kinds of other types of coupling means can be provided ator near the free end of the gangway and on the mooring element. Insteadof gangway it is also possible to use other types of docking arms alongor via which the transfer of persons and/or cargo can take place afterdocking.

Thus the invention provides for an effective, user-friendly and savetransfer system with which persons and all kinds of cargo can be quicklytransferred from a transfer vessel towards a stationary or floatingoffshore object even at heavy sea or otherwise difficult conditions.

It should be understood that various changes and modifications to thepresently preferred embodiments can be made without departing from thescope of the invention, and therefore will be apparent to those skilledin the art. It is therefore intended that such changes and modificationsbe covered by the appended claims.

We claim:
 1. An offshore transfer system for transferring persons and/orcargo between a transfer vessel and an offshore construction, floatingplatform or other vessel, wherein the transfer vessel comprises: adocking position with a mooring element, wherein the offshoreconstruction, floating platform or other vessel comprises: a dockingarm, in particular a gangway, that at a free end is provided with acoupling device, wherein the mooring element is configured to have thecoupling device at the free end of the docking arm coupled theretoduring a docking operation, wherein the docking position on the transfervessel has its mooring element at least partly compensated for transfervessel motions, at least during the docking operation, and comprises:one or more force actuators for moving the mooring element relative tothe transfer vessel; one or more sensors for detecting one or more ofpitch, roll and/or heave motions of the transfer vessel; and a controlunit for driving the one or more force actuators such that the one ormore detected motions of the transfer vessel get at least partlycompensated for the mooring element of the docking position.
 2. Theoffshore transfer system according to claim 1, wherein the mooringelement is supported by and connected to the transfer vessel by means ofa hinge connection designed to allow the transfer vessel to rotaterelative to the mooring element in a rotation direction around a y-axis,wherein the y-axis extends substantially horizontal, and wherein a firstone of the one or more force actuators acts between the transfer vesseland the mooring element for swivelling the mooring element around thehinge connection relative to the transfer vessel at least in therotation direction around said y-axis.
 3. The offshore transfer systemaccording to claim 2, wherein the hinge connection is designed to allowthe transfer vessel to also rotate relative to the mooring element in arotation direction around an x-axis, wherein the x- and y-axis extendsubstantially horizontal and orthogonal relative to each other, andwherein a second one of the one or more force actuators acts between thetransfer vessel and the mooring element for swivelling the mooringelement around the hinge connection relative to the transfer vessel atleast in the rotation direction around said x-axis.
 4. The offshoretransfer system according to claim 3, wherein the hinge connectioncomprises two substantially horizontal orthogonal pivot pins.
 5. Theoffshore transfer system according to claim 4, wherein the two pivotpins form part of a Universal joint, Cardan joint or a Gimbal assembly.6. The offshore transfer system according to one claim 1, wherein themooring element is supported by and connected to the transfer vessel bymeans of a z-guidance designed to allow the transfer vessel to moverelative to the mooring element in a translation direction along az-axis, wherein the z-axis extends substantially vertical, and wherein athird one of the one or more force actuators acts between the transfervessel and the mooring element for translating the mooring element alongthe z-guidance relative to the transfer vessel at least in thetranslation direction along said z-axis.
 7. The offshore transfer systemaccording to claim 1, wherein the mooring element is supported by andconnected to the transfer vessel by means of a x- and/or y-guidancedesigned to allow the transfer vessel to move relative to the mooringelement in a translation direction along an x- and/or y-axis, whereinthe x- and/or y-axis extend substantially horizontal and orthogonalrelative to each other, and wherein a fourth and/or fifth one of the oneor more force actuators acts between the transfer vessel and the mooringelement for translating the mooring element along the x- and/ory-guidance relative to the transfer vessel at least in the translationdirection along said x- and/or y-axis.
 8. The offshore transfer systemaccording to claim 1, wherein the docking position further comprises: anupright column extending upwards from the transfer vessel, wherein themooring element is provided on an upper portion of the column.
 9. Theoffshore transfer system according to claim 8, wherein lever arms areprovided which are fixedly connected to the column and extend in asubstantially sideways direction from it, the force actuators beingprovided in between the transfer vessel and the lever arms.
 10. Theoffshore transfer system according to claim 1, wherein the one or moresensors comprise: a position sensor for sensing deteriorations from anaimed orientation of the mooring element, wherein the control unit isconfigured for driving the force actuators in dependence of senseddeteriorations by the position sensor such that the force actuatorsbring the mooring element back to its aimed orientation.
 11. Theoffshore transfer system according to claim 1, wherein the forceactuators are hydraulic cylinders.
 12. The offshore transfer systemaccording to claim 1, wherein the docking arm is telescopinglyextendable, wherein drive means are provided for retracting or extendingthe docking arm during mooring, and/or wherein the docking arm ismounted on the offshore construction, floating platform or other vesselmovable around a horizontal and/or vertical shaft, wherein drive meansare provided for retracting or extending the docking arm during adocking operation.
 13. A transfer vessel for an offshore transfer systemaccording to claim 1, comprising a mooring element that is at leastpartly compensated for transfer vessel motions.
 14. A method forcoupling the transfer vessel with the docking arm on the offshoreconstruction, floating platform or other vessel, according to one of thepreceding claims, comprising the steps: sailing the transfer vessel to aposition close to the offshore construction, floating platform or othervessel; and coupling the coupling device at the free end of the dockingarm to the mooring element on the transfer vessel, wherein during and/orafter the coupling has been made, the force actuators are operated forgetting or keeping the mooring element positioned in an aimedorientation independent of motions of the transfer vessel.